Apparatus and method for treating fuel vapor of an engine

ABSTRACT

A fuel vapor treatment apparatus controls fuel vapor produced in a fuel tank supplied to an engine intake system to prevent dispersion into the atmosphere. It involves adjusting a purge air quantity to a high accuracy, to thereby avoid the occurrence of air-fuel ratio fluctuations between cylinders due to the purge air. More specifically, with a construction wherein the purge air quantity is adjusted by duty controlling the opening/closing of a purge passage, the purge air quantity is set based on the engine load and engine rotational speed, and the control frequency used in the duty control is increasingly changed corresponding to an increase in purge air quantity. Moreover, the opening/closing of the purge passage is duty controlled based on the purge air quantity and the control frequency.

TECHNICAL FIELD

The present invention relates to an apparatus and method for treatingfuel vapor of an engine. In particular, the invention relates totechnology for supplying fuel vapor produced inside a fuel tank, to theengine intake system to thereby prevent diffusion into the atmosphere.

BACKGROUND ART

Apparatus for treating fuel vapor to prevent fuel vapor inside a fueltank from diffusing into the atmosphere (referring to JapaneseUnexamined Patent Publication No. 62-7962), have involved temporarilyabsorbing the fuel vapor produced inside the fuel tank into a canister,and then purging the absorbed fuel vapor and supplying the purge airthrough a purge passage to the engine intake system.

With such an apparatus the purge air from a canister is supplied to theengine intake system, with the normal supply mixture. As a fixedquantity of purge air is supplied irrespective of the engine operatingconditions, there is the likelihood of a large discrepancy in theair-fuel ratio.

To compensate, the quantity of purge air is changed in accordance withengine operating conditions such as engine load. To achieve this, thepurge air quantity is adjusted by duty controlling the ON/OFF switchingof a purge control solenoid valve that drives a valve for opening andclosing the purge passage, according to a duty ratio corresponding tothe purge flow quantity required for the operating conditions.

However, with the duty control of the purge control solenoid valve, whenthe frequency of the duty control is low, purge air pulsations occurwith the opening/closing of the purge passage. As a result, variationsoccur in the purge air quantity drawn into the respective cylinderscausing variations in the air-fuel ratio between the cylinders.

So that the purge air is drawn in uniformly into the respectivecylinders, the frequency of the duty control should be as high aspossible. However, if a high frequency is used, then the power supplytime becomes shorter so that the flow quantity region, where there is adrop in control accuracy due to the valve opening delay time, isenlarged compared to when a low frequency is used. As a result, highaccuracy flow quantity adjustment cannot be made in the low flowquantity region.

More specifically, while the solenoid valve has a constant valve openingdelay time, when duty control is carried out at a high frequency, thepower supply time (valve opening time) is shortened compared to that forcontrol at a low frequency, so that the proportion of the power supplytime control region occupied by the valve opening delay time isincreased. Therefore, when adjusting a low flow quantity at a highfrequency, the power supply time within the valve opening delay timeduring which the air quantity is unstable is given, making it difficultto adjust a low flow quantity to a high accuracy.

Moreover, with a construction wherein a high frequency that can reliablyavoid purge air pulsations is used indiscriminately, since the ON/OFFswitching per unit time is increased significantly, then there is thelikelihood of deterioration in the life of the valve seat components.

DISCLOSURE OF THE INVENTION

In view of the above situation, it is an object of the presentinvention, with a construction wherein the purge air quantity isadjusted by a duty control, to suppress variations in purge air intakequantity between cylinders, so as to avoid a deterioration in flowquantity control accuracy, and a deterioration in the life ofcomponents.

Accordingly, with the apparatus and method according to the presentinvention for treating the fuel vapor of an engine incorporating acanister for absorbing the fuel vapor in a fuel tank, purging theabsorbed fuel vapor, and supplying the purge air to the engine intakesystem by way of a purge passage. The purge air quantity supplied to theengine intake system is set based on engine operating conditions, whilethe control frequency at the time of duty controlling theopening/closing of the purge passage is set in accordance with the setpurge air quantity. The opening/closing of the purge passage is thenduty controlled based on the set purge air quantity and controlfrequency.

With such a construction, the purge air quantity is controlled to anappropriate value corresponding to the engine operating conditions byduty controlling the opening/closing of the purge passage, the dutycontrol being carried out at different control frequencies in accordancewith the purge air quantity. By changing the control frequency inaccordance with the purge air quantity, the variations in purge airintake quantity between the cylinders can be suppressed, so that flowquantity adjustment accuracy can be maintained.

The construction may be such that the frequency of the duty controlsignal is increasingly set in accordance with an increase in the purgeair quantity.

With such a construction, when the purge air quantity is small, theopening/closing of the purge passage can be duty controlled at arelatively low frequency to thereby maintain the adjustment accuracy ofthe purge air quantity, while when the purge air quantity is large, theopening/closing of the purge passage can be controlled at a relativelyhigh frequency to thereby avoid the variations in purge air intakequantity between the cylinders. The occurrence of air-fuel ratiovariations between the cylinders can thus be prevented.

Moreover, the construction may be such that the engine load and enginerotational speed are detected, and the purge air quantity is set basedon the detected engine load and engine rotational speed.

With such a construction, the occurrence of changes in air-fuel ratiodue to the addition of purge air to the normal mixture can besuppressed, so that deterioration in operability due to the supply ofpurge air can be avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a basic construction of an apparatusfor treating fuel vapor of an engine, according to the presentinvention;

FIG. 2 is a system diagram showing an embodiment of the presentinvention;

FIG. 3 is a flow chart showing a canister purge control routine in theembodiment;

FIGS. 4A and 4B are graphs showing aspects of duty control at highfrequency; and

FIG. 5A and 5B are graphs showing aspects of duty control at lowfrequency.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows the basic construction of an apparatus for treating fuelvapor of an engine, according to the present invention.

In FIG. 1, a purge passage open/close device A constitutes a device foropening and closing a purge passage, while an operating conditionsdetection device B constitutes a device for detecting the engineoperating conditions.

Moreover, a purge air quantity setting device C sets a purge airquantity supplied to the engine intake system, based on engine operatingconditions detected by the operating conditions detection device B,while a duty control device D duty controls the opening/closing of thepurge passage open/close device A in accordance with the set purge airquantity.

A control frequency varying device E variably sets the frequency of aduty control signal of the duty control device D in accordance with thepurge air quantity set by the purge air quantity setting device C,

More specifically, the construction is such that the frequency of theduty control at the time of duty controlling the opening/closing thepurge passage is not fixed, but is changed in accordance with the purgeair quantity at the time, so that the purge passage open control timeproportion in the control frequency is changed.

Embodiments of an apparatus and method for treating the fuel vapor of anengine, according to the present invention will now be described.

FIG. 2 shows a construction of an embodiment of an apparatus. With thisembodiment, air is drawn into an engine 1 by way of a throttle chamber 2and intake manifold 3.

The throttle chamber 2 is provided with a throttle valve 4 linked to anaccelerator pedal (not shown), for controlling the intake air quantityto the engine 1.

Solenoid type fuel injection valves 5 are provided for each cylinder inrespective branch portions of the intake manifold 3 for injecting fuel,which has been pressurized by a fuel pump (not shown), and controlled toa predetermined pressure by a pressure regulator, to inside the intakemanifold 3.

The fuel injection valves 5 are intermittently driven open in responseto an injection pulse signal from a control unit 6 incorporating amicrocomputer, and the fuel injection quantity is controlledcorresponding to a pulse width of the injection pulse signal computed bythe control unit 6.

Respective ignition plugs 7 are provided for each cylinder of theengine 1. A high voltage generated by an ignition coil 8 is appliedsuccessively to the ignition plugs 7 by way of a distributor 9 tothereby cause a spark to ignite the mixture. Here the high voltagegeneration period of the ignition coil 8 is controlled by means of apower transistor 10 attached thereto.

Fitted to the throttle valve 4 is a throttle sensor 11, which detectsthe opening (TVO) by means of a potentiometer.

Detection signals for each predetermined crank angle are output from acrank angle sensor 12 provided inside the distributor 9. The enginerotational speed Ne can thus be computed based on the detection signals.

A water temperature sensor 13 for detecting a cooling water temperatureTw, is provided in the cooling water jacket of the engine 1, while anoxygen sensor 15 for detecting the concentration of oxygen in theexhaust gases, which concentration is closely related to the air-fuelratio of the intake mixture of the engine 1, is provided in the exhaustmanifold 14. Moreover an air flow meter 33 for detecting an intake airquantity Qa of the engine 1, is provided in the intake duct upstream ofthe throttle chamber 2.

The engine 1 is furnished with a vapor treatment apparatus 21 forpreventing the diffusion of fuel vapor produced inside a fuel tank 20,into the atmosphere.

With the vapor treatment apparatus 21, fuel vapor produced inside thefuel tank 20 is temporarily absorbed into an absorption agent 23 such asactivated carbon which is filled into a canister 22. The fuel that hasbeen absorbed into the absorption agent 23 is then purged, and the purgeair supplied through a purge passage 24 to the intake passage downstreamof the throttle valve 4.

Fuel vapor inside the fuel tank 20 is introduced to the canister 22 byway of a vapor passage where a check valve 25 is disposed, which valveis opened when a positive pressure inside the fuel tank 20 is largerthan or equal to a predetermined value. Moreover, disposed in the purgepassage 24 is a diaphragm valve 28 incorporating a pressure chamber intowhich negative throttle pressure or atmospheric pressure are selectivelyintroduced by way of a reference pressure passage 27.

With the diaphragm valve 28, when a negative throttle pressure isapplied to the pressure chamber, the purge passage 24 is opened inopposition against the urging force of a valve closing spring 28a. Whenthe pressure chamber is at atmospheric pressure, the urging force of thevalve closing spring 28a closes the valve, thereby closing the purgepassage 24.

A purge control solenoid valve 29 which is duty controlled ON and OFF bythe control unit 6, is disposed in the reference pressure passage 27,for selectively applying a negative throttle pressure to the pressurechamber of the diaphragm valve 28.

The construction of the purge control solenoid valve 29 is such thatwhen in an OFF condition (open), a negative pressure passage 30, whichintroduces a negative throttle pressure, is connected to the referencepressure passage 27, to give a large purge air quantity, while when inan ON condition (closed), an atmospheric pressure passage 31 forintroducing atmospheric pressure from upstream of the throttle valve 4,is connected to the reference pressure passage 27, to give a small purgeair quantity.

Accordingly, the negative throttle pressure and atmospheric pressure canbe alternately introduced to the pressure chamber of the diaphragm valve28, by ON/OFF switching of the purge control solenoid valve 29. By dutycontrolling this ON/OFF switching, the purge air quantity adjusted bymeans of the diaphragm valve 28 can be variably controlled. In thisrespect, the purge control solenoid valve 29 corresponds to the purgepassage open/close device of the embodiment.

Aspects of a canister purge control carried out by the control unit 6will now be described in accordance with the flow chart of FIG. 3.

In the present embodiment, the functions of a purge air quantity settingdevice, a duty control device, and a control frequency varying device,are realized by software illustrated by the flow chart of FIG. 3 andstored in the control unit 6. Moreover, the operating conditionsdetection device corresponds to the air flow meter 33 and the crankangle sensor 12.

In the flow chart of FIG. 3, initially in step 1 (with step denoted by Sin FIG. 3), a map in which a basic purge rate PA has been pre-storedwith parameters of engine load Tp (Tp=K×Qa/Ne; where K is a constant),which is computed based on intake air quantity Qa and engine rotationalspeed Ne, and engine rotational speed Ne is looked up to obtain a basicpurge ratio PA corresponding to the current engine load Tp androtational speed Ne.

Then in step 2, the engine intake air quantity Qa is read, and in step 3the basic purge ratio PA is changed using the basic purge ratio PA, theintake air quantity Qa, and a conversion coefficient KP, into a purgeflow quantity PG (PG=Qa×PA×KP). Hence with this embodiment, the purgeflow quantity PG is determined so that the quantity of purge airsupplied is a predetermined proportion of the intake air quantity Qa.

In step 4, a control frequency fv for when the ON/OFF switching of thepurge control solenoid valve 29 is duty controlled, is determined basedon the set purge flow quantity PC. Here the lower the purge flowquantity PG the lower the control frequency fv.

Since there is the possibility of pulsations of the purge air when thecontrol frequency fv is set comparatively low, then preferably from thepoint of view of avoiding the purge pulsations, the frequency should beas high as possible.

However, with a construction wherein duty control is carried out at ahigh frequency, then as shown in FIGS. 4A and 4B, due to the shorteningof the power supply time, the influence of the valve opening delay timebecomes significant so that flow quantity adjustment accuracy drops.Moreover, with the construction wherein duty control is normally carriedout at a high frequency, since the ON/OFF switching frequency of thesolenoid per unit time is increased, the life of the valve seatcomponents becomes a problem.

With the present embodiment, with a low flow quantity, the accuracy offlow quantity adjustment is given priority over the avoidance of purgeair pulsations. Therefore duty control is made at low frequency (referto FIGS. 5A and 4B). Since with the low flow quantity the purge airquantity is small, then even in the event of purge air pulsations, anyvariation in air-fuel ratio between cylinders due to the purge airpulsations will be minimal, so that in this respect duty control at thelow frequency presents no problem, or rather the effect that variationsin the purge air quantity can be avoided by duty control at a lowfrequency is important.

With a high flow quantity, the occurrence of pulsations in the purge aircan be avoided by high frequency control (see FIG. 4A and 4B), so thatthe quantity of purge air drawn into the respective cylinders can bemade uniform. Moreover with a high flow quantity, the influence of thevalve opening delay of the purge control solenoid valve 29 becomesminimal, so that flow quantity adjustment accuracy can also bemaintained.

However, when duty control is carried out indiscriminately at a highfrequency, the ON/OFF switching frequency by the duty control issignificantly increased. Therefore to avoid carrying out duty controlunnecessarily at a high frequency, the control frequency is graduallyincreased in accordance with the increase in required flow quantity. Asa result, deterioration in the life of the valve seat components due tohigh frequency duty control can be minimized.

Once the duty control frequency fv has been set as mentioned above, withthe frequency being lower the lower the purge flow quantity PG, then inthe next step 5, a control duty P_(DUTY) for the purge control solenoidvalve 29 required to obtain the purge flow quantity, is computed asfollows:

    P.sub.DUTY =(P.sub.G /Kv)×(1/fv)

In the above equation for computing the control duty P_(DUTY), K_(V) isthe maximum purge flow quantity.

Once the control duty P_(DUTY) has been computed, control proceeds tostep 6 where the control duty P_(DUTY) is Output to the purge controlsolenoid valve 29 at a frequency fv. As a result, the negative throttlepressure introduced to the diaphragm valve 28 is regulated, therebyadjusting the purge air quantity supplied to the engine via thediaphragm valve 28.

In the above embodiment, the frequency fv is variably set based only onthe purge flow quantity PG required for the engine operating conditions.However, in priority to this frequency setting, in the injectioninterval for the fuel injection valves 5 set by the engine rotationalspeed Ne, the frequency fv may be set so that the ON/OFF switchingfrequency of the purge control solenoid valve 29 is maintained at thelowest limit. As a result the occurrence of air-fuel ratio variationsbetween the cylinders due to the purge pulsations can be reliablyavoided.

With the vapor treatment apparatus 21 of the present embodiment, theconstruction is such that the purge passage 24 is opened/closed by meansof a diaphragm type valve. However, a construction is also possiblewherein the purge passage 24 is opened and closed directly with asolenoid valve.

INDUSTRIAL APPLICABILITY

With the present invention as described above, the construction is suchthat the frequency of the duty control is changed in accordance with thepurge air quantity, at the time of duty controlling the opening/closingof the purge passage. Therefore, the accuracy of adjusting the purge airquantity can be maintained so that the occurrence of large fluctuationsin the purge air quantity drawn into the respective cylinders, due tothe pulsations in the purge air can be avoided.

Accordingly the invention has significant industrial applicability.

I claim:
 1. An apparatus for treating fuel vapor of an engineincorporating a canister for absorbing the fuel vapor in a fuel tank,purging the absorbed fuel vapor, and supplying the purge air to anengine intake system by way of a purge passage, said apparatuscomprising:purge passage open/close means for opening and closing thepurge passage; operating conditions detection means for detecting engineoperating conditions; purge air quantity setting means for setting apurge air quantity supplied to the engine intake system based on theengine operating conditions detected by said operating conditiondetection means; duty determining means for determining opening/closingduty of said purge passage open/close means in accordance with the purgeair quantity set by said purge air quantity setting means; frequencydetermining means for determining a frequency of opening/closing of saidpurge passage open/close means in accordance with the purge air quantityset by said purge air quantity setting means to suppress variations inpurge air intake quantity between cylinders; and duty control means foroutputting duty control signals of said determined duty and frequency tosaid purge passage open/close means to duty control the opening/closingof said purge passage open/close means.
 2. An apparatus for treatingfuel vapor of an engine according to claim 1, wherein said frequencydetermining means increasingly sets a frequency of the duty controlsignal in accordance with an increase in the purge air quantity.
 3. Anapparatus for treating fuel vapor of an engine according to claim 1,wherein said operating conditions detections means detects the engineload and engine rotational speed, and said purge air quantity settingmeans sets a purge air quantity based on the detected engine load andengine rotational speed.
 4. A method of treating fuel vapor of an engineincorporating a canister for absorbing the fuel vapor in a fuel tank,purging the absorbed fuel vapor, and supplying the purge air to anengine intake system by way of a purge passage, said method includingthe steps of:detecting engine operating conditions; setting a purge airquantity supplied to the engine intake system based on the detectedengine operating conditions; determining opening/closing duty of thepurge passage based on the set purge air quantity; setting a frequencyof opening/closing of said purge passage based on the set purge airquantity to suppress variations in purge air intake quantity betweencylinders; and controlling the opening/closing of said purge passagebased on the set purge air quantity and frequency.
 5. A method oftreating fuel vapor of an engine according to claim 4, wherein theopening/closing frequency of the purge passage increases with anincrease in the purge air quantity.
 6. A method of treating fuel vaporof an engine according to claim 4, wherein said step for detecting theengine operating conditions detects the engine load and enginerotational speed, and said step for setting a purge air quantity sets apurge air quantity based on the engine load and engine rotational speed.